Nuclear magnetic resonance study of promoted catalysts
Norcross, James Andrew
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Permalink
https://hdl.handle.net/2142/19892
Description
Title
Nuclear magnetic resonance study of promoted catalysts
Author(s)
Norcross, James Andrew
Issue Date
1991
Doctoral Committee Chair(s)
Slichter, C.P.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
We have used NMR to study the properties of promoted catalysts prepared by adding alkali metals (Na, K, Rb, or Cs) to silica supported Pt clusters. We have also examined the effect of the alkali metals on CO and C$\sb{2}$H$\sb{4}$ adsorbed on the Pt. The properties of the promoter were studied in samples containing $\sp{23}$Na. By performing a $\sp{23}$Na-$\sp{13}$C doubled resonance with adsorbed CO, we determined that at least 22% of the NA in the samples is on the Pt. The $\sp{23}$Na line shows no indication of a Knight shift and the Na spin-lattice relaxation varies as 1/T$\sp2$, suggesting that the Na orbitals mix with the Pt conduction band to a much lesser degree than do those of adsorbed CO. Consideration of several results leads us to conclude that the Na forms three dimensional structures on the Pt, most likely Na$\sb{2}$O molecules or crystallites. We observed the effect of the promoters Na, K, Rb, and Cs on the NMR properties of adsorbed CO. In all cases the effects are small. The CO line is narrowed and shifted to higher frequency in the presence of the promoter while the CO spin lattice relaxation is slowed. The effect of the promoter is greater in the samples with the heavier alkali metals K, Rb, and Cs. In the case of CO and Rb, for which the effect of the alkali metal is greatest, a modest change of about 10-15% in the parameters used to describe the NMR results is adequate to account for the effect of the Rb. We investigated C$\sb{2}$H$\sb{\rm 4}$ adsorbed on Pt both with and without K. By performing $\sp{13}$C-$\sp{1}$H double resonance we discovered that the C$\sb{2}$H$\sb{4}$ exists as a CCH$\sb{3}$ species on the surface in both samples. The results of an annealing experiment, conducted to study C-C bond scission, indicate about a 2% reduction in the activation energy for C-C bond scission for C$\sb{2}$H$\sb{4}$ coadsorbed with K.
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